The very first thing I do after doing a clean Install of any macOS is to make sure not to turn on iCloud. I'm not so afraid of using it but I know from previous experience that any services that Apple provides over the internet can be obsoleted by them at any time without notice so I prefer to be safe than sorry. I don't like being left in a lurch by them just because they've decided they don't want to support something anymore, and since they have zero communications skills when it comes to notifying their customers when this is about to happen I don't trust them any further than I can throw Apple Park.
yes couldn't agree more when it come to installing a new MacOS version (even if not a beta) - a very good practice to make sure everything is backed up and "thought out" before turning on iCloud and to consider each option in the "iCloud Drive" options - which can be turned on / off separately.
I have had "new install" issues in the past with my Contacts not syncing properly and loosing more than 1000 contacts, also had issues with iPhoto migration to Photos and using photos on iCloud (lost a lot of pictures due to poor sync and stopped using photos on iCloud)
Also from painful experience I always avoid turning anything on or off in iCloud to see if that fixes things like contact sync, iMessage issues, Mail, reminders, calendar, keychain etc.
Is anyone else afraid to use it and is there any negative to turning it off and staying away from it?
iCloud things I don't use - (keep a local copy and a back up)
Photos - use local storage and CCC to back up weekly
Desktop Documents Folders - easy to just create custom folders (and back up the iCloud drive folder weekly)
Apple Music - too confusing to Apple Streaming content from Local Files and Playlists
iCloud things that work great - IMHO
custom folders - iCloud drive - Documents and Files to iPhone 8 and iPP 12.9
iMessages - use extensively for work communication with clients - have almost 4g or messages
Contacts - sync always seems buggy but works - also have duplicated in Outlook just in case
Reminders - works great
Calendars - works great except for outlook invites
Keychain - works great - but also use a Password Manager